Nickel-cobalt-manganese composite hydroxide, and production method therefor

ABSTRACT

Achieved is a nickel-cobalt-manganese composite hydroxide which is excellent in reactivity with a lithium compound, and able to achieve a positive electrode active material which has excellent thermal stability and battery characteristics. The nickel-cobalt-manganese composite hydroxide is intended to serve as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery, and represented by a general formula: Ni1-x-y-zCoxMnyMz(OH)2 (0&lt;x≤⅓, 0&lt;y≤⅓, 0≤z≤0.1, M represents one or more elements selected from Mg, Al, Ca, Ti, V, Cr, Zr, Nb, Mo, and W), and the nickel-cobalt-manganese composite hydroxide has a specific surface area of 3.0 to 11.0 m2/g as measured by a BET method through nitrogen adsorption, and an average valence of 2.4 or more for Co and Mn as obtained by redox titration.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a nickel-cobalt-manganese composite hydroxide which serves as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery, and a production method therefor, and more particularly, relates to a nickel-cobalt-manganese composite hydroxide which serves as a precursor for a positive electrode active material of a lithium ion secondary battery, and a production method therefor. The present application claims priority based on the Japanese Patent Application No. 2013-126893 filed on Jun. 17, 2013 in Japan, which is incorporated by reference herein.

Description of Related Art

Conventionally, smaller and lighter secondary batteries which have high energy densities have been required with popularization of mobile devices such as cellular phones and laptop personal computers. Batteries preferred for these applications include lithium ion secondary batteries, which have been actively researched and developed.

Furthermore, also in a field of automobiles, a demand for electric vehicles has been increased from resource and environmental perspectives, and lithium ion secondary batteries which are small and light, high in discharging capacity, with favorable cycle characteristics have been required as power sources for electric vehicles and hybrid automobiles. In particular, for power sources for automobiles, output characteristics are important, and lithium ion secondary batteries have been required which have favorable output characteristics.

Lithium ion secondary batteries that use, for positive electrode active materials, lithium-containing composite oxides, in particular, relatively easily synthesized lithium-cobalt composite oxides (LiCoO₂) achieve high voltages on the order of 4 V grade, and have been thus progressively put into practical use as batteries with high energy densities. Further, a large number of lithium ion secondary batteries that use this type of lithium-cobalt composite oxide have ever been developed for achieving excellent initial capacity characteristics and cycle characteristics, and various results have been already achieved.

However, the lithium-cobalt composite oxides cause, because of the use of expensive cobalt compounds for raw materials, increases in the cost of active materials and thus batteries, and improved active materials have been desired. The battery that uses the lithium-cobalt composite oxide has a significantly higher unit price per capacity than a nickel-hydrogen battery, and thus has a considerably limited use application. Therefore, there are great expectations to lower the cost of active materials, thereby allowing the manufacture of more inexpensive lithium ion secondary batteries not only small-size secondary batteries for currently popular portable devices, but also for large-size secondary batteries, e.g., for electricity storage, and for electric vehicles, and the achievement of the manufacture can be considered to have industrially enormous significance.

In this regard, 4 V-grade positive electrode active materials that are more inexpensive than lithium-cobalt composite oxides, that is, lithium-nickel-cobalt-manganese composite oxides that have a composition of Li[Ni_(1/3)Co_(1/3)Mn_(1/3)]O₂ with nickel:cobalt:manganese substantially of 1:1:1 in atomic ratio have been attracting attention as new materials-of positive electrode active materials for lithium ion secondary batteries. The lithium-nickel-cobalt-manganese composite oxides have been actively developed, because of not only their inexpensiveness, but also because of exhibiting higher thermal stability than lithium ion secondary batteries that use a lithium-cobalt composite oxide or a lithium-nickel composite oxide for a positive electrode active material.

In order for lithium ion secondary batteries to provide favorable battery characteristics, the lithium-nickel-cobalt-manganese composite oxides as positive electrode active materials need to have moderate particle sizes and specific surface areas, and also have high densities. These properties of the positive electrode active materials strongly reflect the properties of nickel-cobalt-manganese composite hydroxides as precursors, and similar properties are thus required for the composite hydroxides.

Furthermore, in order to obtain positive electrode active materials that provide favorable battery characteristics, nickel-cobalt-manganese composite hydroxides are required which are likely to cause reactions with lithium compounds to proceed even in a water vapor or a carbon dioxide gas generated during syntheses with the lithium compounds, and excellent in reactivity. Nickel-cobalt-manganese composite hydroxides which are poor in reactivity with lithium compounds make reactions with nickel-cobalt-manganese composite hydroxides incomplete during syntheses with the lithium compounds, thereby producing residual unreacted lithium compounds. In addition, there is the problem of melting the lithium compounds, thereby causing agglomeration, before completing the reactions between the nickel-cobalt-manganese composite hydroxides and the lithium compounds.

In regard to nickel-cobalt-manganese composite hydroxides to serve as precursors for positive electrode active materials, various suggestions have been made as will be described below. However, while the increase in density has been examined in each of the suggestions, surface properties of the nickel-cobalt-manganese composite hydroxides or the reactivity thereof with lithium compounds have not been taken into full account.

For example, Patent Literature 1 proposes a method of continuously supplying an aqueous solution of a nickel salt, which contains a cobalt salt and a manganese salt, a complexing agent, and an alkali metal hydroxide, in an inert gas atmosphere or in the presence of a reductant in a reaction vessel, and continuously extracting crystals obtained through continuous crystal growth. Patent Literature 1 mentions obtaining a spherical high-density cobalt-manganese co-precipitated nickel hydroxide that has a tap density of 1.5 g/cm³ or more, an average particle size of 5 to 20 μm, and a specific surface area of 8 to 30 m²/g.

The co-precipitated nickel hydroxide obtained can be used as a raw material for lithium-nickel-cobalt-manganese composite oxides. However, this co-precipitated nickel hydroxide has, according to an example, a tap density of 1.71 to 1.91 g/cm³, which is less than 2.0 g/cm³, which can be thus considered to be an insufficiently high density. On the other hand, any specific numerical value is not mentioned for the specific surface area, it is not clear whether the specific surface area is made appropriate or not, and the reactivity with lithium compounds has not been examined. Therefore, even when this co-precipitated nickel hydroxide is used as a precursor, any lithium-nickel-cobalt-manganese composite oxide will not be obtained which has favorable battery characteristics.

In addition, Patent Literature 2 suggests a method for producing a lithium-nickel-cobalt-manganese composite oxide, which includes the steps of: reacting and co-precipitating mixed aqueous solution of a nickel salt, a cobalt salt, and a manganese salt with nickel:cobalt:manganese of substantially 1:1:1 in atomic ratio in the presence of a complexing agent in an aqueous solution, with pH of 9 to 13 with an alkali solution under an inert gas atmosphere, thereby providing a nickel-cobalt-manganese composite hydroxide and/or a nickel-cobalt-manganese composite oxide with nickel:cobalt:manganese of substantially 1:1:1 in atomic ratio; and firing a mixture of the hydroxide and/or the oxide with a lithium compound at 700° C. or higher so that the atomic ratio of lithium to the total atomic ratio of nickel, cobalt, and manganese is substantially 1:1.

Also in accordance with the method proposed in Patent Literature 2, the obtained nickel-cobalt-manganese composite hydroxide has a tap density of 1.95 g/cm³, which is less than 2.0 g/cm³, and a very large specific surface area of 13.5 m²/g. Moreover, the reactivity with lithium compounds has not been examined.

Accordingly, nickel-cobalt-manganese composite hydroxides have been desired which have favorable reactivity with lithium compounds, and make it possible to produce nickel-cobalt-manganese composite oxides such that favorable battery characteristics are achieved.

PRIOR ART Patent Documents

Patent document 1: Japanese Laid-Open Patent Publication 2008-195608

Patent document 2: Japanese Laid-Open Patent Publication 2003-59490

BRIEF SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Therefore, an object of the present invention is to provide a nickel-cobalt-manganese composite hydroxide which is excellent in reactivity with a lithium compound, and is able to achieve a positive electrode active material for a non-aqueous electrolyte secondary battery which has excellent thermal stability and battery characteristics, and a method for producing the nickel-cobalt-manganese composite hydroxide.

Means to Solve the Problem

The present inventors have, in order to solve the problems mentioned above, earnestly researched the influences of nickel-cobalt-manganese composite hydroxides on the reactivity with lithium compounds to obtain a finding that the reactivity is greatly affected by the average valence for Co and Mn and specific surface area through oxidation after drying, thereby completing the present invention.

The nickel-cobalt-manganese composite hydroxide according to the present invention for achieving the object mentioned above is a nickel-cobalt-manganese composite hydroxide represented by a general formula: Ni_(1-x-y-z)Co_(x)Mn_(y)M_(z)(OH)₂ (0<x≤⅓, 0<y≤⅓, 0≤z≤0.1, M represents one or more elements selected from Mg, Al, Ca, Ti, V, Cr, Zr, Nb, Mo, and W), to serve as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery, the nickel-cobalt-manganese composite hydroxide is characterized by having a specific surface area of 3.0 to 11.0 m²/g as measured by a BET method through nitrogen adsorption, and an average valence of 2.4 or more for Co and Mn as obtained by redox titration.

The method for producing a nickel-cobalt-manganese composite hydroxide according to the present invention for achieving the object mentioned above is a method for producing the nickel-cobalt-manganese composite hydroxide, which is characterized by including: a crystallization step of mixing, in a reaction vessel, a mixed aqueous solution containing at least a nickel salt, a cobalt salt, and a manganese salt, and an aqueous solution containing an ammonium ion supplier, and supplying a aqueous solution of caustic alkali so that the pH on the basis of a liquid temperature of 25 degrees is kept in the range of 11 to 13, thereby providing a reaction solution, and crystallizing nickel-cobalt-manganese composite hydroxide particles in the reaction solution in a non-oxidizing atmosphere or in the presence of a reductant; a solid-liquid separation step of applying solid-liquid separation and water washing to the nickel-cobalt-manganese composite hydroxide particles; a drying step of drying, in a non-oxidizing atmosphere, the nickel-cobalt-manganese composite hydroxide particles subjected to the water washing; and an oxidation step of oxidizing the dried nickel-cobalt-manganese composite hydroxide particles, thereby providing a specific surface area of 3.0 to 11.0 m²/g as measured by a BET method through nitrogen adsorption, and an average valence of 2.4 or more for Co and Mn as obtained by redox titration.

Effects of Invention

The present invention is able to provide a nickel-cobalt-manganese composite hydroxide to serve as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery, which is excellent in reactivity with a lithium compound, and provide a non-aqueous electrolyte secondary battery which has excellent thermal stability and also excellent battery characteristics, when a lithium-nickel-cobalt-manganese composite oxide obtained with the nickel-cobalt-manganese composite hydroxide as a precursor is used as a positive electrode active material of a non-aqueous electrolyte secondary battery. Furthermore, the present invention is able to easily produce nickel-cobalt-manganese composite hydroxides on an industrial scale, and extremely high in industrial value.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph of a sample obtained after increasing, under a carbon dioxide gas atmosphere, the temperature of a mixture of a nickel-cobalt-manganese composite hydroxide obtained in Example 1 and a lithium compound.

FIG. 2 is a scanning electron micrograph of a sample obtained after increasing, under a carbon dioxide gas atmosphere, the temperature of a mixture of a nickel-cobalt-manganese composite hydroxide obtained in Comparative Example 1 and a lithium compound.

DETAILED DESCRIPTION OF THE INVENTION

A nickel-cobalt-manganese composite hydroxide and a production method therefor to which the present invention is applied will be described in detail below. It is to be noted that the present invention is not limited to the following detailed description, unless otherwise stated. Embodiments according to the present invention will be described in the following order:

-   1. Nickel-Cobalt-Manganese Composite Hydroxide -   2. Method for Producing Nickel-Cobalt-Manganese Composite Hydroxide     -   2-1. Crystallization Step     -   2-2. Solid-Liquid Separation Step     -   2-3. Drying Step     -   2-4. Oxidation Step

<1. Nickel-Cobalt-Manganese Composite Hydroxide>

The nickel-cobalt-manganese composite hydroxide according to the present embodiment serve as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery, in particular, as a precursor for a positive electrode active material of a lithium ion secondary battery. The nickel-cobalt-manganese composite hydroxide represented by the general formula: Ni_(1-x-y-z)Co_(x)Mn_(y)M_(z)(OH)₂ (0<x≤⅓, 0<y≤⅓, 0≤z≤0.1, M represents one or more elements selected from Mg, Al, Ca, Ti, V, Cr, Zr, Nb, Mo, and W), serves as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery. The nickel-cobalt-manganese composite hydroxide has a specific surface area of 3.0 to 11.0 m²/g as measured by a BET method through nitrogen adsorption, and an average valence of 2.4 or more for Co and Mn as obtained by redox titration.

When the specific surface area exceeds 11.0 m²/g, the specific surface area of a positive electrode active material finally obtained is excessively increased, thereby failing to achieve adequate safety. Alternatively, when the specific surface area is less than 3.0 m²/g, the reactivity with a lithium compound is degraded in the synthesis with the lithium compound, thereby causing the reaction to fail to proceed sufficiently, and in an elevated temperature process, the lithium compound is melted, thereby causing agglomeration. Accordingly, in order to increase the safety of the positive electrode active material, and prevent agglomeration, the specific surface area of the nickel-cobalt-manganese composite hydroxide is adapted to fall within the range of 3.0 to 11.0 m²/g.

When the average valence for Co and Mn as obtained by redox titration is less than 2.4, due to the fact that the synthesis with a lithium compound is an oxidation reaction, not only the reactivity with the lithium compound is degraded, thereby causing the reaction to fail to proceed sufficiently with the lithium compound remaining in some cases, but also in an elevated temperature process, the lithium compound is fused, thereby causing agglomeration between particles. When the average valence is increased more than necessary, the oxidation of the nickel-cobalt-manganese composite hydroxide may proceed, thereby excessively increasing the specific surface area. For this reason, the average valence is preferably adjusted to 2.4 to 3.3.

Furthermore, the nickel-cobalt-manganese composite hydroxide preferably has an average mesopore radius of 4.00 to 6.00 nm and a pore volume of 0.010 to 0.020 ml/g as measured by a nitrogen adsorption method. The nickel-cobalt-manganese composite hydroxide which meets these requirements has more favorable reactivity with a lithium compound, and achieves sufficient reactivity with the lithium compound.

With the average mesopore radius less than 4.00 nm, or the pore volume less than 0.010 ml/g, insufficient penetration of lithium carbonate melted into particles in the synthesis may decrease the reactivity with the lithium compound. On the other hand, with the average mesopore radius in excess of 6.00 nm or the pore volume in excess of 0.020 ml/g, the specific surface area may be excessively increased, thereby failing to achieve adequate safety.

The nickel-cobalt-manganese composite hydroxide is preferably 5 to 15 μm in average particle size, and preferably 2.0 g/cm³ or more in tap density. The nickel-cobalt-manganese composite hydroxide which meets these requirements can further improve battery performance, in the case of use as a precursor of a positive electrode active material.

The additive element represented by M in the general formula is added to improve battery characteristics such as cycle characteristics and output characteristics. When the atomic ratio z of the additive element M exceeds 0.1, the battery capacity is unfavorably decreased because of the reduced metal element contributing to the Redox reaction. Accordingly, the additive element M is adjusted to be 0.1 or less in atomic ratio z.

The foregoing nickel-cobalt-manganese composite hydroxide is preferred as a precursor of a positive electrode active material of a non-aqueous electrolyte secondary battery, and can be made into a positive electrode active material of a non-aqueous electrolyte secondary battery by a common manufacturing method.

For example, as a method for producing the nickel-cobalt-manganese composite oxide which serves as a positive electrode active material of a lithium ion secondary battery, first, the nickel-cobalt-manganese composite hydroxide is left as it is, or subjected to heat treatment at a temperature of 800° C. or lower. Next, a lithium compound is mixed so that the lithium is 0.95 to 1.5 in atomic ratio with respect to the metal element of the nickel-cobalt-manganese composite hydroxide, and subjected to firing at 800 to 1000° C. Thus, the nickel-cobalt-manganese composite oxide can be obtained.

The above-described nickel-cobalt-manganese composite hydroxide has a specific surface area of 3.0 to 11.0 m²/g as measured by a BET method through nitrogen adsorption, and an average valence of 2.4 or more for Co and Mn as obtained by redox titration, the nickel-cobalt-manganese composite hydroxide is thus excellent in reactivity with a lithium compound and likely to cause the reaction with the lithium compound to proceed, and the reaction with the lithium compound proceeds preferentially rather than the melting of the lithium compound, thereby making it possible to prevent agglomeration from being caused.

Thermogravimetric measurement and differential thermal analysis (TG-DTA) in an inert atmosphere can be used as methods for evaluating the nickel-cobalt-manganese composite hydroxide for the reactivity with the lithium compound and ease of agglomeration. For example, a mixture of the nickel-cobalt-manganese composite hydroxide and lithium carbonate can be evaluated by TG-DTA measurement in a carbon dioxide gas atmosphere. With the degraded reactivity of the nickel-cobalt-manganese composite hydroxide with the lithium carbonate, the unreacted lithium carbonate is left after elevating the temperature, and the DTA curve has an endothermic peak derived from the melting of the lithium carbonate, which is observed around 680 to 720° C. In addition, the sample after the TG-DTA measurement can be subjected to observation by SEM to confirm whether the nickel-cobalt-manganese composite hydroxide is agglomerated or not.

The foregoing nickel-cobalt-manganese composite hydroxide is excellent in reactivity with a lithium compound, and kept from being agglomerated. When the nickel-cobalt-manganese composite hydroxide is used as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery as just described, a non-aqueous electrolyte secondary battery can be obtained which has excellent battery characteristics while being excellent in thermal stability.

<2. Method for Producing Nickel-Cobalt-Manganese Composite Hydroxide>

Next, a method will be described for producing the nickel-cobalt-manganese composite hydroxide described above.

In the method for producing the nickel-cobalt-manganese composite hydroxide, a reaction solution is obtained by mixing a mixed aqueous solution containing at least a nickel salt, a cobalt salt, and a manganese salt with an aqueous solution containing an ammonium ion supplier, and supplying an aqueous solution of caustic alkali so that the pH on the basis of a liquid temperature of 25° C. is maintained within the range of 11 to 13, and nickel-cobalt-manganese composite hydroxide particles are crystallized in the reaction solution, dried after solid-liquid separation, and oxidized after the drying. This production method can increase the specific surface area of the nickel-cobalt-manganese composite hydroxide and the average valence for Co and Mn thereof.

Each step will be described in detail below in regard to the method for producing the nickel-cobalt-manganese composite hydroxide.

(2-1) Crystallization Step

In the crystallization step, a reaction solution is obtained by mixing a mixed aqueous solution containing at least a nickel salt, a cobalt salt, and a manganese salt with an aqueous solution containing an ammonium ion supplier, and supplying an aqueous solution of caustic alkali so that the pH on the basis of a liquid temperature of 25° C. is maintained within the range of 11 to 13, and nickel-cobalt-manganese composite hydroxide particles are crystallized in the reaction solution in a non-oxidizing atmosphere or in the presence of a reductant.

In the crystallization step, the temperature of the reaction solution is preferably maintained at 20 to 70° C. This grows crystals of the nickel-cobalt-manganese composite hydroxide. When the temperature of the reaction solution is lower than 20° C., the low solubility of the salt in the reaction solution lowers the salt concentration, thus failing to sufficiently grow crystals of the nickel-cobalt-manganese composite hydroxide. On the other hand, when the temperature of the reaction solution exceeds 70° C., the generation of many crystal nuclei results in many fine particles, thus failing to provide a high density of nickel-cobalt-manganese composite hydroxide particles.

In addition, in the crystallization step, the reaction solution is controlled so that the pH on the basis of a liquid temperature of 25° C. falls within the range of 11 to 13, preferably 11 to 12. The pH less than 11 makes particles of the nickel-cobalt-manganese composite hydroxide coarse, and moreover, after the reaction, causes nickel to remain in the reaction solution, thereby leading to a loss of nickel. On the other hand, the pH in excess of 13 increases the crystallization rate of the nickel-cobalt-manganese composite hydroxide, thereby resulting in many fine particles. In the case of resulting in many fine particles, a problem is caused which is that the particles are made sintered, thereby producing an agglomerated powder.

The pH of the reaction solution can be controlled by supplying an aqueous solution of caustic alkali. The aqueous solution of caustic alkali is not particularly limited, but for example, an alkali metal hydroxide aqueous solution such as sodium hydroxide or potassium hydroxide can be used. The alkali metal hydroxide can be also directly added to the reaction solution, but is preferably added as an aqueous solution from the perspective of ease of pH control. The method of adding the aqueous solution of caustic alkali is not particularly limited, but while adequately stirring the reaction solution, the aqueous solution of caustic alkali may be added with a pump that is able to control the flow rate, such as a metering pump, so that the pH on the basis of a liquid temperature of 25° C. falls within the range of 11 to 13.

Furthermore, in the crystallization step, nickel-cobalt-manganese composite hydroxide particles are generated by co-precipitation in a non-oxidizing atmosphere or in the presence of a reductant. In the case of crystallization in an oxidizing atmosphere or in the absence of a reductant, the cobalt and the manganese are oxidized, thereby making new nuclei likely to be generated, and excessively increasing the specific surface area due to fine primary particles. Moreover, the density of secondary particles is lowered, thus causing excessive increases in average pore radius and pore volume.

In addition, in the crystallization step, a non-oxidizing atmosphere is preferably achieved by maintaining the oxygen concentration in the atmosphere in a reaction vessel in contact with an open side of the reaction solution at 0.2 volume % or less. When the oxygen concentration is maintained at 0.2 volume % or less, the oxidation of the metal elements, in particular, manganese in the reaction solution is suppressed to develop primary particles, and thus obtain highly crystalline spherical secondary particles, thereby making it possible to prevent any small specific surface area and excessive increases in average pore radius and pore volume.

When the oxygen concentration exceeds 0.2 volume %, primary particles are made fine, thereby increasing crystal interfaces, and the nickel-cobalt-manganese composite hydroxide obtained may thus have a specific surface area in excess of 11.0 m²/g. Moreover, the average pore radius and the pore volume may be excessively increased. The lower limit of the oxygen concentration is not particularly limited, but the specific surface area and the carbon content are also lowered with decreasing oxygen concentration, and the oxygen concentration may be thus adjusted within the range of 0.2 volume % or less and an industrially possible range to allow control to a desired specific surface area and carbon content. The carbon content is preferably 0.1 mass % or less in the nickel-cobalt-manganese composite hydroxide. The carbon content can be measured by a high frequency-infrared combustion method.

In order to adjust the oxygen concentration in the reaction vessel to 0.2 volume % or less, for example, an inert gas is preferably used because it is easy to control the concentration by supplying an inert gas into the reaction vessel. In this regard, for the supply of an inert gas into the reaction vessel, there is a need to supply or to continuously supply a sufficient amount of inert atmosphere gas during the crystallization so that the oxygen concentration is maintained at 0.2 volume % or less. Therefore, the adjustment of the oxygen concentration can be easily made by, for example, using a reaction vessel with a lid, and adjusting the amount of inert gas supplied to the reaction vessel. As the inert gas, a nitrogen gas is advantageous in terms of cost, and preferred.

It is to be noted that the adjustment is not limited to the control method mentioned above, because the oxygen concentration has only to be retained at 0.2 volume % or less.

In the crystallization step, even when the oxygen concentration is not retained at 0.2 volume % or less, it is possible to control the specific surface area and pores with primary particle developed and secondary particles increased in size, as long as the addition of a reductant to the reaction solution suppresses oxidation of the metal elements. The reductant may be added to the extent that the oxidation of the metal element is suppressed, and may be any reductant that is able to suppress the oxidation of the metal element, such as hydrazine.

The nickel-cobalt-manganese composite hydroxide obtained in the crystallization step, which is represented by the general formula: Ni_(1-x-y-z)Co_(x)Mn_(y)M_(z)(OH)₂ (0<x≤⅓, 0<y≤⅓, 0≤z≤0.1, M represents one or more elements selected from Mg, Al, Ca, Ti, V, Cr, Zr, Nb, Mo, and W), give close agreement with the atomic ratios in the raw material supplied. Therefore, the atomic ratios of the nickel, cobalt, manganese, and additive element M can be adjusted to fall within the ranges in the above general formula by adjusting the atomic ratios in the raw material to the atomic ratios in the general formula.

Specifically, the salt concentration of the mixed aqueous solution of the nickel salt, cobalt salt, and manganese salt is preferably adjusted to 1 mol/L to 2.6 mol/L in total for the respective salts. In the case of less than 1 mol/L, the low salt concentration results in failure to cause crystals of the nickel-cobalt-manganese composite hydroxide to grow sufficiently. On the other hand, in the case of greater than 2.6 mol/L, which is greater than the saturating concentration at ordinary temperature, there is a risk such as clogging a pipe with redeposition of crystals, and the generation of many crystal nuclei results in many fine particles.

The nickel salt, the cobalt salt, and the manganese salt are not particularly limited, but preferably at least one of sulfates, nitrates, or chlorides.

The ammonium ion supplier for use in the crystallization step is not particularly limited, but preferably at least one of ammonia, ammonium sulfate, or ammonium chloride.

The additive amount of the ammonium ion supplier preferably falls within the range of 5 to 20 g/L in ammonium ion concentration in the reaction solution. In the case of less than 5 g/L in ammonium ion concentration, the low solubility of the nickel, cobalt, and manganese in the reaction solution results in insufficient crystal growth, thus failing to achieve any high-density nickel-cobalt-manganese composite hydroxide. Alternatively, in excess of 20 g/L in ammonium ion concentration, the crystallization rate is decreased, thereby degrading productivity, and more metal ions such as nickel remaining in the solution lead to an increase in cost.

The additive element M, which has one or more elements selected from Mg, Al, Ca, Ti, V, Cr, Zr, Nb, Mo, and W, can be added to the mixed aqueous solution during the crystallization step, or added individually to the reaction solution to provide the nickel-cobalt-manganese composite hydroxide with the composition of the general formula.

The additive element M is preferably added as a water-soluble compound, and for example, titanium sulfate, ammonium peroxotitanate, titanium potassium oxalate, vanadium sulfate, ammonium vanadate, chromium sulfate, potassium chromate, zirconium sulfate, zirconium nitrate, niobium oxalate, ammonium molybdate, sodium tungstate, ammonium tungstate, etc. can be used.

When the additive element M is uniformly dispersed within the nickel-cobalt-manganese composite hydroxide particles, an additive containing the additive element M may be added to the mixed aqueous solution, and co-precipitation can be achieved while the additive element M is uniformly dispersed within the nickel-cobalt-manganese composite hydroxide particles.

In addition, as a method of adding the additive element M, the surfaces of the nickel-cobalt-manganese composite hydroxide particles may be coated with the additive element M. In such a case, for example, in a way that the nickel-cobalt-manganese composite hydroxide particles are made into slurry with an aqueous solution containing the additive element M, and while controlling to predetermined pH, an aqueous solution containing one or more additive elements M is added to deposit the additive elements M on the surfaces of the nickel-cobalt-manganese composite hydroxide particles through a crystallization reaction, the surfaces can be coated uniformly with the additive elements M. In place of the aqueous solution containing the additive element M, an alkoxide solution of the additive element M may be used. Furthermore, the surfaces of the nickel-cobalt-manganese composite hydroxide particles can be also coated with the additive element M by spray-drying an aqueous solution or slurry containing the additive element M onto the nickel-cobalt-manganese composite hydroxide particles. In addition, the surfaces can be also coated by a method such as spraying and drying slurry in which the nickel-cobalt-manganese composite hydroxide particles and a salt containing one or more additive elements M are suspended, or mixing the nickel-cobalt-manganese composite hydroxide and a salt containing one or more additive elements M by a solid-phase method.

It is to be noted that in the case of coating the surfaces of the nickel-cobalt-manganese composite hydroxide particles with the additive element M, the ratio of the atom number of additive element ions present in the mixed aqueous solution is reduced by the amount of coating, thereby making it possible to give agreement with the desired ratio of the atom number of metal ions of the nickel-cobalt-manganese composite hydroxide particles.

The reaction mode in the crystallization step is not particularly limited, but a batch mode or a continuous mode can be used. The continuous mode is a mode in which the mixed aqueous solution, an aqueous solution containing the ammonium ion supplier, and an aqueous solution of caustic alkali are each continuously supplied to continuously overflow, from the reaction vessel, a reaction solution containing nickel-cobalt-manganese composite hydroxide particles, thereby collecting the nickel-cobalt-manganese composite hydroxide particles, and preferred in terms of productivity and stability.

In the case of the continuous mode, it is preferable to, while keeping the temperature constant, supply certain amounts of the mixed aqueous solution and ammonium ion supplier to the reaction vessel, control the pH through the addition of the aqueous solution of caustic alkali, and continuously sample generated particles with an overflow pipe after the reaction vessel reaches a steady state therein. It is to be noted that while it is also possible to mix the mixed aqueous solution and the aqueous solution of caustic alkali in advance, and then supply the mixture to the reaction vessel, it is preferable to supply the mixed aqueous solution and the aqueous solution of caustic alkali individually to the reaction vessel in order to prevent the nickel-cobalt-manganese composite hydroxide from being produced in the mixed aqueous solution in mixing with the aqueous solution of caustic alkali.

In the case of using any of the reaction modes, sufficient stirring is preferred in order to maintain a homogeneous reaction during the crystallization. In addition, it is preferable to use water with the impurity content reduced as much as possible, such as pure water, for the water for use in the crystallization step in order to prevent impurities from being mixed.

(2-2) Solid-Liquid Separation Step

In the solid-liquid separation step, the nickel-cobalt-manganese composite hydroxide particles obtained by the crystallization can be filtered, then washed with water, and again filtered to collect the nickel-cobalt-manganese composite hydroxide particles. The filtration may be a commonly used method, for example, a centrifuge or a suction filtration device can be used. In addition, the water washing may be any commonly used method, as long as the method can remove the excess base and non-reducing complexing agent included in the nickel-cobalt-manganese composite hydroxide particles. For the water for use in the water washing, in order to prevent impurities from being mixed, it is preferable to use water with the impurity content reduced as much as possible, and it is more preferable to use pure water.

(2-3) Drying Step

In the drying step, the nickel-cobalt-manganese composite hydroxide particles subjected to the solid-liquid separation is dried in a non-oxidizing atmosphere, preferably at a drying temperature of 100 to 230° C.

In the case of drying in an oxidizing atmosphere, it is difficult to control the oxidation, and during the drying, the oxidation of the nickel-cobalt-manganese composite hydroxide particles further proceed to excessively increase the specific surface area. Moreover, the pore volume is excessively increased. For this reason, it becomes difficult to control the specific surface area and pores in the oxidation step as a subsequent step.

The non-oxidizing atmosphere may be any atmosphere as long as the atmosphere can suppress the oxidation of the nickel-cobalt-manganese composite hydroxide particles, and is preferably an inert gas atmosphere such as a vacuum atmosphere or a nitrogen atmosphere with an oxygen concentration of 0.2 volume % or less.

The drying temperature is the object temperature, that is, the maximum temperature of the nickel-cobalt-manganese composite hydroxide particles dried. Water evaporation is insufficient when the drying temperature is lower than 100° C., while the decomposition of the nickel-cobalt-manganese composite hydroxide proceeds to turn into a mixture with oxides when the drying temperature exceeds 230° C. When the oxides are present, the content of metals such as nickel per mass varies depending on the mixed amount of the oxides, thus making it difficult to combine the hydroxide accurately with a lithium compound in the step of producing a positive electrode active material, and making it difficult to achieve adequate battery characteristics from a positive electrode active material obtained.

(2-4) Oxidation Step

In the oxidation step, the dried nickel-cobalt-manganese composite hydroxide particles are oxidized. In the oxidation step, the nickel-cobalt-manganese composite hydroxide particles have a specific surface area of 3.0 to 11.0 m²/g as measured by a BET method through nitrogen adsorption, and an average valence of 2.4 or more for Co and Mn as obtained by redox titration. In the oxidation step, the nickel-cobalt-manganese composite hydroxide particles are oxidized to generate fine cracks in the particles, thereby increasing the specific surface area. Thus the reactivity is improved between the nickel-cobalt-manganese composite hydroxide particles and a lithium compound. In addition, the previously mentioned fine cracks are finer than the pores formed during the crystallization, and thus, the average pore radius is decreased, whereas the increased number of pores increases the pore volume.

Therefore, the oxidation condition is determined appropriately so as to achieve desired specific surface area and average valence. The oxidation temperature is preferably adjusted to 20 to 230° C. At the oxidation temperature lower than 20° C., the oxidation will not proceed, and the specific surface area may be less than 3.0, or the average valence for Co and Mn may be less than 2.4. In addition, the low oxidation temperature is industrially unfavorable, because the oxidation time is excessively long. Moreover, the average pore radius and the pore volume may be excessively reduced. On the other hand, when the oxidation temperature exceeds 230° C., the oxidation will proceed rapidly, and the specific surface area may thus exceed 11.0 m²/g. In addition, the decomposition of the nickel-cobalt-manganese composite hydroxide may proceed to provide a mixture with oxides. Moreover, the average pore radius and the pore volume may be excessively increased.

The oxidation time may be adjusted to be long when the oxidation temperature is low, or to be short when the oxidation temperature is high, so as to achieve desired specific surface area and average valence. In addition, the atmosphere for the oxidation may be any atmosphere that oxidizes Co and Mn, and is preferably an air atmosphere in terms of controllability.

As just described, in the method for producing the nickel-cobalt-manganese composite hydroxide, a reaction solution is obtained by mixing a mixed aqueous solution containing at least a nickel salt, a cobalt salt, and a manganese salt with an aqueous solution containing an ammonium ion supplier, and supplying an aqueous solution of caustic alkali so that the pH on the basis of a liquid temperature of 25° C. is maintained within the range of 11 to 13, nickel-cobalt-manganese composite hydroxide particles are crystallized in the reaction solution, dried after solid-liquid separation, and oxidized after the drying.

This method for producing a nickel-cobalt-manganese composite hydroxide can achieve a nickel-cobalt-manganese composite hydroxide represented by the general formula: Ni_(1-x-y-z)Co_(x)Mn_(y)M_(z)(OH)₂ (0<x≤⅓, 0<y≤⅓, 0≤z≤0.1, M represents one or more elements selected from Mg, Al, Ca, Ti, V, Cr, Zr, Nb, Mo, and W), which has a specific surface area of 3.0 to 11.0 m²/g as measured by a BET method through nitrogen adsorption, and an average valence of 2.4 or more for Co and Mn as obtained by redox titration.

The obtained nickel-cobalt-manganese composite hydroxide has crystals moderately made fine, a specific surface area increased, and an average valence of 2.4 or more for Co and Mn, and thus serves as a precursor for a positive electrode active material, from which a non-aqueous electrolyte secondary battery can be obtained which has high reactivity with a lithium compound and has excellent thermal stability and battery characteristics.

Therefore, a non-aqueous electrolyte secondary battery, in particular, a lithium ion secondary battery that uses a positive electrode active material produced from the obtained nickel-cobalt-manganese composite hydroxide has a high capacity and favorable cycle characteristics, and has excellent battery characteristics and safety.

EXAMPLES

While the present invention will be described in more detail below with reference to examples and comparative examples, the present invention is not limited by the examples in any way. Further, here are evaluation methods for nickel-cobalt-manganese composite hydroxides and positive electrode active materials for non-aqueous electrolyte secondary batteries, which were used in the examples and the comparative examples.

(1) Analysis of Metal Component:

Analyses were made by ICP emission spectrometry with the use of an ICP (Inductively Coupled Plasma) emission spectrometer (from VARIAN, 725ES).

(2) Analysis of Ammonium Ion Concentration:

The concentration was measured by a distillation method in accordance with the JIS standards.

(3) Measurement of BET Specific Surface Area

The surface area was measured by a single-point BET method through nitrogen adsorption with the use of a specific surface area measuring instrument (from Yuasa Ionics Co., Ltd, Multisorb 16).

(4) Measurement of Average Particle Size and Evaluation of Particle Size Distribution Width:

The measurement of average particle sizes and the evaluation of particle size distribution widths were made with the use of a laser diffraction particle size distribution meter (from Nikkiso Co., Ltd., Microtrac

(5) TG-DTA Measurement:

Thermogravimetric measurements and differential thermal analyses were made with the use of a differential-type differential thermal balance (from Bruker AXS, TG-DTA2020SR).

(6) Observation and Evaluation of Morphology:

The observation of shapes and appearances and the evaluation on the presence or absence of agglomeration were made with the use of a scanning electron microscope (from JEOL Ltd., JSM-6360LA, hereinafter, referred to as a SEM).

Example 1

In Example 1, 4 L of pure water and 200 mL of 25 mass % ammonia water were put in an overflow-type crystallization reaction vessel of 5 L in tank volume with four baffles attached, and heated to 60° C. by a thermostatic bath and a heating jacket, and the pH of the reaction solution in the thermostatic bath was adjusted to 12.0 on the basis of a liquid temperature of 25° C. with the addition of a 25 mass % caustic soda solution.

Next, while stirring the reaction solution maintained at 60° C., a metering pump was used to continuously supply a mixed aqueous solution of: nickel sulfate with a nickel concentration of 0.667 mol/L; cobalt sulfate with a cobalt concentration of 0.667 mol/L; and manganese sulfate with a manganese concentration of 0.667 mol/L (hereinafter, referred to as a mixed aqueous solution) at 10 ml/min, and also 25 mass % ammonia water at 1.5 ml/min, with the addition of a 25 mass % caustic soda solution, thereby developing a crystallization reaction by control of the pH on the basis of a liquid temperature of 25° C. to 12.0 and of the ammonium ion concentration to 5 to 15 g/L.

The stirring for the crystallization was carried out by horizontal rotation at a rotation speed of 800 rpm with the use of six turbine blades of 8 cm in diameter. In addition, as a method for supplying the mixed aqueous solution into the reaction system, an injection nozzle to serves as a supply port was inserted into the reaction solution in such a way that the mixed aqueous solution was supplied directly into the reaction solution.

The nickel-cobalt-manganese composite hydroxide particles generated by the crystallization reaction were continuously extracted through overflow. The particles were extracted over 48 to 72 hours from the start of the reaction when the reaction was stabilized. The nickel-cobalt-manganese composite hydroxide particles were subjected to solid-liquid separation with the use of a Buchner funnel and a suction bottle, and then washed with water to obtain a filtered product. This filtered product was dried by keeping the product at a drying temperature of 120° C. for 12 hours in a vacuum dryer.

Next, this dried product was oxidized by maintaining the product for 1 hour in a stationary dryer kept at 120° C., thereby providing a nickel-cobalt-manganese composite hydroxide.

In the obtained nickel-cobalt-manganese composite hydroxide, the grade of nickel was 21.5 wt %, the grade of cobalt was 21.5 wt %, and the grade of manganese was 19.8 wt %, the proportions of the respective elements were 33.6:33.4:33.0 nearly equal to the composition ratios of the raw material aqueous solution, and the average particle size was 10.5 μm. In addition, the specific surface area was 7.6 m²/g, and the average valence for Co and Mn was 3.1. These values are shown in Table 1.

The obtained nickel-cobalt-manganese composite hydroxide was mixed with a lithium carbonate, and then, when the mixture was subjected to a TG-DTA measurement up to 980° C. at a rate of temperature increase of 5° C./min in a carbon dioxide gas atmosphere at 100 ml/min, no endothermic peak was observed between 680 and 720° C. In addition, when the sample subjected to the TG-DTA measurement was subjected to grinding in a mortar, and observed by a SEM, no agglomeration was confirmed. Table 1 shows the presence or absence of an endothermic peak between 680 and 720° C. and the presence or absence of agglomeration observed by the SEM. In addition, FIG. 1 shows a scanning electron micrograph of the sample obtained according to Example 1. Furthermore, Table 1 shows the results of measuring the average mesopore radius and the pore volume by a nitrogen adsorption method for the nickel-cobalt-manganese composite hydroxide.

Example 2

In Example 2, a nickel-cobalt-manganese composite hydroxide was obtained and subjected to each evaluation in the same way as in Example 1 except that the dried product was oxidized by maintaining the product for 2 hours in a stationary dryer. The specific surface area of the nickel-cobalt-manganese composite hydroxide was 8.6 m²/g, and the average valence for Co and Mn was 3.2.

The obtained nickel-cobalt-manganese composite hydroxide was mixed with a lithium carbonate, and then, when the mixture was subjected to a TG-DTA measurement up to 980° C. at a rate of temperature increase of 5° C./min in a carbon dioxide gas atmosphere at 100 ml/min, no endothermic peak was observed between 680 and 720° C. In addition, when the sample subjected to the TG-DTA measurement was subjected to grinding in a mortar, and observed by a SEM, no agglomeration was confirmed. Table 1 shows the presence or absence of an endothermic peak between 680 and 720° C. and the presence or absence of agglomeration observed by the SEM. Furthermore, Table 1 shows the results of measuring the average mesopore radius and the pore volume by a nitrogen adsorption method for the nickel-cobalt-manganese composite hydroxide.

Example 3

In Example 3, a nickel-cobalt-manganese composite hydroxide was obtained and subjected to each evaluation in the same way as in Example 1 except that the dried product was oxidized by maintaining the product further for 3 weeks at 25° C. in a stationary dryer. The specific surface area of the nickel-cobalt-manganese composite hydroxide was 5.2 m²/g, and the average valence for Co and Mn was 2.5.

The obtained nickel-cobalt-manganese composite hydroxide was mixed with a lithium carbonate, and then, when the mixture was subjected to a TG-DTA measurement up to 980° C. at a rate of temperature increase of 5° C./min in a carbon dioxide gas atmosphere at 100 ml/min, no endothermic peak was observed between 680 and 720° C. In addition, when the sample subjected to the TG-DTA measurement was subjected to grinding in a mortar, and observed by a SEM, no agglomeration was confirmed. Table 1 shows the presence or absence of an endothermic peak between 680 and 720° C. and the presence or absence of agglomeration observed by the SEM. Furthermore, Table 1 shows the results of measuring the average mesopore radius and the pore volume by a nitrogen adsorption method for the nickel-cobalt-manganese composite hydroxide.

[Comparative Example 1]

In Comparative Example 1, a nickel-cobalt-manganese composite hydroxide was obtained and subjected to each evaluation in the same way as in Example 1 except that the dried product was not maintained in a stationary dryer, that is, not oxidized. The specific surface area of the nickel-cobalt-manganese composite hydroxide was 2.8 m²/g, and the average valence for Co and Mn was 2.1.

The obtained nickel-cobalt-manganese composite hydroxide was mixed with a lithium carbonate, and then, when the mixture was subjected to a TG-DTA measurement up to 980° C. at a rate of temperature increase of 5° C./min in a carbon dioxide gas atmosphere at 100 ml/min, an endothermic peak was observed between 680 and 720° C. In addition, when the sample subjected to the TG-DTA measurement was subjected to grinding in a mortar, and observed by a SEM, agglomeration was confirmed. Table 1 shows the presence or absence of an endothermic peak between 680 and 720° C. and the presence or absence of agglomeration observed by the SEM. In addition, FIG. 2 shows a scanning electron micrograph of the sample obtained according to Comparative Example 1. Furthermore, Table 1 shows the results of measuring the average mesopore radius and the pore volume by a nitrogen adsorption method for the nickel-cobalt-manganese composite hydroxide.

[Comparative Example 2]

In Comparative Example 2, a nickel-cobalt-manganese composite hydroxide was obtained and subjected to each evaluation in the same way as in Example 1 except that the dried product was maintained for further 10 minutes in a stationary dryer. The specific surface area of the nickel-cobalt-manganese composite hydroxide was 3.6 m²/g, and the average valence for Co and Mn was 2.3.

The obtained nickel-cobalt-manganese composite hydroxide was mixed with a lithium carbonate, and then, when the mixture was subjected to a TG-DTA measurement up to 980° C. at a rate of temperature increase of 5° C./min in a carbon dioxide gas atmosphere at 100 ml/min, an endothermic peak was observed between 680 and 720° C. In addition, when the sample subjected to the TG-DTA measurement was subjected to grinding in a mortar, and observed by a SEM, agglomeration was confirmed. Table 1 shows the presence or absence of an endothermic peak between 680 and 720° C. and the presence or absence of agglomeration observed by the SEM. Furthermore, Table 1 shows the results of measuring the average mesopore radius and the pore volume by a nitrogen adsorption method for the nickel-cobalt-manganese composite hydroxide.

[Comparative Example 3]

In Comparative Example 3, a nickel-cobalt-manganese composite hydroxide was obtained and subjected to each evaluation in the same way as in Example 1 except that the dried product was maintained for further 5 hours in a stationary dryer. The specific surface area of the nickel-cobalt-manganese composite hydroxide was 11.3 m²/g, and the average valence for Co and Mn was 3.4.

The obtained nickel-cobalt-manganese composite hydroxide was mixed with a lithium carbonate, and then, when the mixture was subjected to a TG-DTA measurement up to 980° C. at a rate of temperature increase of 5° C./min in a carbon dioxide gas atmosphere at 100 ml/min, no endothermic peak was observed between 680 and 720° C. In addition, when the sample subjected to the TG-DTA measurement was subjected to grinding in a mortar, and observed by a SEM, no agglomeration was confirmed. Table 1 shows the presence or absence of an endothermic peak between 680 and 720° C. and the presence or absence of agglomeration observed by the SEM. Furthermore, Table 1 shows the results of measuring the average mesopore radius and the pore volume by a nitrogen adsorption method for the nickel-cobalt-manganese composite hydroxide.

[Comparative Example 4]

In Comparative Example 4, a nickel-cobalt-manganese composite hydroxide was obtained and subjected to each evaluation in the same way as in Example 1 except that the filtered product was maintained for 12 hours at a drying temperature of 120° C. in a stationary dryer under an air atmosphere, but the dried product after the drying was not oxidized. The specific surface area of the nickel-cobalt-manganese composite hydroxide was 11.9 m²/g, and the average valence for Co and Mn was 3.4.

The obtained nickel-cobalt-manganese composite hydroxide was mixed with a lithium carbonate, and then, when the mixture was subjected to a TG-DTA measurement up to 980° C. at a rate of temperature increase of 5° C./min in a carbon dioxide gas atmosphere at 100 ml/min, no endothermic peak was observed between 680 and 720° C. In addition, when the sample subjected to the TG-DTA measurement was subjected to grinding in a mortar, and observed by a SEM, no agglomeration was confirmed. Table 1 shows the presence or absence of an endothermic peak between 680 and 720° C. and the presence or absence of agglomeration observed by the SEM. Furthermore, Table 1 shows the results of measuring the average mesopore radius and the pore volume by a nitrogen adsorption method for the nickel-cobalt-manganese composite hydroxide.

TABLE 1 Oxidation Specific CoMn Endothermic Reaction Temp- Surface Average Average Pore Peak from Solution Drying erature Oxidation Area Valence Mesopore Volume 680 to Agglomer- (pH) Atmosphere (° C.) Time (m²/g) (valence) Size (nm) (ml/g) 720° C. ation Example 1 12.0 Vacuum 120 1 hour 7.6 3.1 4.81 0.018 Absent Absent Example 2 12.0 Vacuum 120 2 hours 8.6 3.2 4.69 0.018 Absent Absent Example 3 12.0 Vacuum  25 3 weeks 5.2 2.5 5.58 0.011 Absent Absent Comparative 12.0 Vacuum — — 2.8 2.1 6.26 0.009 Present Present Example 1 Comparative 12.0 Vacuum 120 10 minutes 3.6 2.3 6.04 0.009 Present Present Example 2 Comparative 12.0 Vacuum 120 5 hours 11.3 3.4 4.30 0.021 Absent Absent Example 3 Comparative 12.0 Air — — 11.9 3.4 4.19 0.021 Absent Absent Example 4

From the results shown in Table 1, according to Examples 1 to 3, the nickel-cobalt-manganese composite hydroxides obtained in a way that nickel-cobalt-manganese composite hydroxide particles crystallized by maintaining the pH of the reaction solution at 12 on the basis of a liquid temperature of 25 degrees were dried, and then oxidized for a predetermined period of time has achieved specific surface areas within the range of 3.0 to 11.0 m²/g and average valences of 2.4 or more for Co and Mn.

It is determined that the nickel-cobalt-manganese composite hydroxide with the specific surface area and average valence for Co and Mn within desired ranges as just described, without any endothermic peak observed between 680 and 720° C., is likely to cause the reaction with the lithium compound to proceed, thereby causing the reaction between the nickel-cobalt-manganese composite hydroxide and a lithium carbonate to proceed preferentially rather than fusion of the lithium carbonate in a carbon dioxide gas atmosphere, without any agglomeration caused as shown in FIG. 1.

From the foregoing, it is determined that the nickel-cobalt-manganese composite hydroxides according to Examples 1 to 3 have high reactivity with the lithium compounds.

On the other hand, it is determined that Comparative Example 1 with the specific surface area of smaller than 3 m²/g and average valence of lower than 2.4 for Co and Mn, with an endothermic peak observed between 680° C. and 720° C., is less likely to cause the reaction with the lithium compound to proceed, thereby causing fusion of a lithium carbonate to proceed preferentially rather than the reaction between the nickel-cobalt-manganese composite hydroxide and the lithium carbonate in a carbon dioxide gas atmosphere, and causing agglomeration as shown in FIG. 2. Accordingly, it is determined that the nickel-cobalt-manganese composite hydroxide according to Comparative Example 1 has low reactivity with the lithium compound.

It is determined that Comparative Example 2 with the average valence of lower than 2.4 for Co and Mn, also with an endothermic peak observed around 680° C. to 720° C. as in Comparative Example 1, is less likely to cause the reaction with the lithium compound to proceed, thereby causing agglomeration.

In addition, in Comparative Example 3, the oxidation excessively proceeded with the longer oxidation time, the specific surface area was thus excessively increased, thereby exceeding 11 m²/g, while the average valence was high. Likewise, in Comparative Example 4, drying the nickel-cobalt-manganese composite hydroxide in the air caused the oxidation to proceed, thereby excessively increasing the specific surface area. 

The invention claimed is:
 1. A nickel-cobalt-manganese composite hydroxide represented by a general formula: Ni_(1-x-y-z)CO_(x)Mn_(y)M_(z)(OH)₂, wherein 0<x≤⅓,0<y≤⅓, 0≤z≤0.1, M represents one or more elements selected from Mg, Al, Ca, Ti, V, Cr, Zr, Nb, Mo, and W, to serve as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery, wherein the nickel-cobalt-manganese composite hydroxide has a specific surface area of 3.0 to 11.0 m²/g as measured by a BET method through nitrogen adsorption, and an average valence of 2.4 or more for Co and Mn as obtained by redox titration, and wherein the nickel-cobalt-manganese composite hydroxide has a property that when mixed with a lithium carbonate thereby forming a mixture, the mixture being subjected to a thermogravimetric measurement and differential thermal analysis (TG-DTA) measurement up to 980° C. at a rate of temperature increase of 5° C/min in a carbon dioxide gas atmosphere at 100 ml/min, has no endothermic peak observed between 680 and 720° C.
 2. The nickel-cobalt-manganese composite hydroxide according to claim 1, wherein the nickel-cobalt-manganese composite hydroxide has an average mesopore radius of 4.00 to 6.00 nm and a pore volume of 0.010 to 0.020 ml/g as measured by a nitrogen adsorption method.
 3. A method for producing a nickel-cobalt-manganese composite hydroxide represented by a general formula: Ni_(1-x-y-z)Co_(x)Mn_(y)M_(z)(OH)₂, wherein 0<x ≤⅓,0<y ≤⅓, 0≤z ≤0.1, M represents one or more elements selected from Mg, Al, Ca, Ti, V, Cr, Zr, Nb, Mo, and W, to serve as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery, the method comprising: a crystallization step of mixing, in a reaction vessel, a mixed aqueous solution containing at least a nickel salt, a cobalt salt, and a manganese salt, and an aqueous solution containing an ammonium ion supplier, and supplying an aqueous solution of caustic alkali so that pH on the basis of a liquid temperature of 25 degrees is kept in a range of 11 to 13, thereby providing a reaction solution, and crystallizing nickel-cobalt-manganese composite hydroxide particles in the reaction solution in a non-oxidizing atmosphere or in the presence of a reductant; a solid-liquid separation step of applying solid-liquid separation and water washing to the nickel-cobalt-manganese composite hydroxide particles; a drying step of drying, in a non-oxidizing atmosphere, the nickel-cobalt-manganese composite hydroxide particles subjected to the water washing; and an oxidation step of oxidizing the dried nickel-cobalt-manganese composite hydroxide particles, thereby providing a specific surface area of 3.0 to 11.0 m²/g as measured by a BET method through nitrogen adsorption, and an average valence of 2.4 or more for Co and Mn as obtained by redox titration.
 4. The method for producing a nickel-cobalt-manganese composite hydroxide according to claim 3, wherein an atmosphere in the reaction vessel in contact with an open side of the reaction solution has an oxygen concentration maintained at 0.2 volume % or less by supplying an inert gas into the reaction vessel in the crystallization step.
 5. The method for producing a nickel-cobalt-manganese composite hydroxide according to claim 3, wherein the reaction solution has a temperature maintained in a range of 20 to 70° C., and an ammonium ion concentration maintained in a range of 5 to 20 g/L.
 6. The method for producing a nickel-cobalt-manganese composite hydroxide according to claim 3, wherein the mixed aqueous solution, the aqueous solution containing the ammonium ion supplier, and the aqueous solution of caustic alkali are each continuously supplied to continuously overflow the reaction solution containing the nickel-cobalt-manganese composite hydroxide particles from the reaction vessel, and collect the nickel-cobalt-manganese composite hydroxide particles.
 7. The method for producing a nickel-cobalt-manganese composite hydroxide according to claim 3, wherein the nickel-cobalt-manganese composite hydroxide particles have surfaces coated with a hydroxide of the additive element M.
 8. The method for producing a nickel-cobalt-manganese composite hydroxide according to claim 3, wherein the nickel salt, the cobalt salt, and the manganese salt are at least one of a sulfate, a nitrate, or a chloride.
 9. The method for producing a nickel-cobalt-manganese composite hydroxide according to claim 3, wherein the ammonium ion supplier is at least one of ammonia, ammonium sulfate, or ammonium chloride.
 10. The nickel-cobalt-manganese composite hydroxide according to claim 1, wherein the average valence is 2.4 to 3.3. 